The invention relates to a method for the manufacture of a ring part out of sheet metal for a lid of a can.
Such a method is described, for example, in the DE 43 32 306 A1. A flat sheet-metal part, which has a hot-sealable coating on one side, is hereby shaped into a cylindrical tube, where the coating is on the outside of the tube. The axially extending edges of the tube are compressed and are welded together, which is done by means of a laser jet and results in the formation of a butt-joint welded seam. The tube is then separated into equal length tube sections or rather into body blanks. Each body blank is reshaped into a ring part, namely on the outside for the formation of an outer annular flange configured to receive the body of the can. A rolling is provided on the inside edge, the upper side of which rolling serves as a bearing surface for mounting of a tear-off foil. This bearing surface has the hot-sealable coating so that the tear-off foil can be sealed with this bearing surface.
This method has a number of disadvantages. The hot-sealable coating, which consists of an organic material, and which is provided on the outside of the tube, is destroyed during the laser welding. Combustion products of the coating get into the melt of the welding seam and cause a carbon accumulation in the welding seam, which in turn causes a hardening of the welding seam. The hardening of the seam is four to five times the hardness of the parent metal. The combustion of the coating takes place like an explosion, which causes holes to be created in the welding seam. In order to guarantee the sealing ability in the welding-seam area, it is necessary to repair the welding-seam area with a new coating. It is furthermore disadvantageous that the high hardness of the welding seam reduces the shaping ability and results in an increased wear of the shaping tools.
The purpose of this invention is to further develop the method so that with an easy shaping ability of the sheet-metal part a hot-sealably coated bearing surface for the tear-off foil is created.